Impact resistant glazing is used for protection from ballistics. In order for a material to be commercially accepted as being effective for ballistic glazing applications, the material must qualify as a “bullet-resistin” material by repeatedly passing an industry-wide certification test procedure. This test is administered by an independent, non-profit organization, Underwriter's Laboratories, Inc. (UL). Underwriter's Laboratories publishes test procedures and performance requirements for bullet-resisting glazing materials in UL-752 (Bullet-Resisting Equipment). There are several levels of increasingly more severe testing under the UL-752 standard, with Level I being the least severe, and increasing levels of severity generally requiring thicker glazing materials, or multilayer composites.
There are several polymeric glazing materials for which impact resistance is asserted, including polyurethanes as described in U.S. Pat. No. 5,962,617, and acrylic sheet as described in U.S. Pat. No. 4,505,972. Sheet thicknesses of at least 1.25 inches of the acrylic copolymer were required to pass a UL-752 (MPSA) or (HPSA) test.
Commercial bullet-resisting sheets are generally very expensive and heavy due to limitations of material strength and transparent requirements. The majority of transparent bullet-resisting sheets are either glass-laminates, glass clad-laminates or plastic-laminates, such as those described in US 2003/0190439 and U.S. Pat. No. 4,594,290. Laminated materials are more complicated and typically more costly to produce than monolithic sheet. The glass laminates are very heavy and tend to shatter when a bullet hits it. The glass clad laminates and the plastic laminates are also very expensive to produce but not quite as heavy to handle.
There is a need for lighter-weight impact-resistant glazing for use in bullet-resistant applications.
Surprisingly it has been found that a monolithic bullet-resistant sheet composed of a copolymer of methyl methacrylate and a C2-18 alkyl methacrylate, containing core-shell impact modifiers provide very good bullet resisting performance. Sheet of the composition of the invention is capable of providing superior impact resistance at current thicknesses, or equal impact resistance at lower thicknesses. The lighter, thinner and stronger sheet compositions of the invention provide increased design flexibility and lower overall cost for bullet resisting applications.